1. Field of the Invention
The present invention relates to an inrush current limiter for a power supply. More precisely, the present invention relates to an inrush current limiter using a non-dissipative thermistor having a power factor correction circuit.
2. Prior Art and Related Information
In many power supplies, especially for computers, there are voltage and current limiters in order to suppress undesirable and spurious current and voltage spikes. Sometimes the voltage spikes are generated by electrical equipment using the same power line, other times the transient surges are caused by the power supply being quickly switched on and off or vice versa. Of course, there are numerous other causes for transient surges known in the art.
The inrush current should be suppressed to avoid damage to the power supply. For example, in many conventional power supplies, there is an input capacitor bank that charges when initially powered up. During this charge-up, the capacitor bank acts as a short circuit and does not limit input current. As a result, the inrush current may activate a circuit breaker or even overload a circuit near the power input.
To avoid this problem, most conventional power supplies include a device to limit the initial input current flowing into the circuit. In a typical low end power supply, in the 50 to 100 watt range, there is a thermistor in series with the input capacitor. As is known in the art, a thermistor is a resistive component having a high negative temperature co-efficient of resistance so that its resistance decreases as its temperature increases. During initial power-up, the thermistor is cold and the resistance is high thereby suppressing unwanted current transients. When the power supply is left on, the temperature of the thermistor rises and its resistance decreases, thus decreasing the energy losses.
In a larger power supply, a thermistor cannot be used because the current load may cause a catastrophic failure of the thermistor. Instead, the thermistor is replaced with a common resistor shunted by a triac or a silicon controlled rectifier (SCR). Using a conventional resistor in place of the thermistor is not completely desirable either. In particular, if the triac or SCR fails, several hundred watts of power is loaded on the resistor causing it to possibly explode, resulting in a catastrophic failure to the power supply.
Thermistors are not used in large power supplies also because they are not accurate devices. Specifically, after the thermistor has heated up, and if the power supply is turned off and on quickly, a transient surge in the current cannot be suppressed because the hot thermistor is still functioning with low resistance.
There have been other attempts at limiting the inrush current. For example, U.S. Pat. No. 3,947,751 to Bray discloses an electronic triac surge current limiting circuit. The circuit limits the inrush current on the AC line, but does not show a boost converter or a power factor boost converter. Bray uses a thermistor as known in the art to limit inrush current.
U.S. Pat. No. 5,122,724 to Criss discloses an inrush current limiter applicable only to low voltage DC to DC converters. Criss limits inrush current by modulating the drive to a field effect transistor (FET), which acts as a resistor during the transition from power off to on. This resistance limits the inrush current during turn on.
U.S. Pat. No. 4,719,553 to Hinckley discloses an inrush current limiter that uses relays for disabling the switching regulator during the initial turn-on phase of the power. There is no mention of use of a thermistor or like device.
U.S. Pat. No. 4,396,882 to Kellenbenz discloses a current limiting device that controls the maximum current or the peak current by limiting the current in a Darlington transistor. Kellenbenz relates to a regular input power supply and does not contemplate a boost DC to DC converter. Further, there is no use of a thermistor. It appears that no thermistor was used because the thermistor is effective only when it is cold, and if a thermistor is put in series with the line when the circuit is turned off and on again, the thermistor cannot perform its surge suppression because it is hot.
U.S. Pat. No. 4,628,431 to Kayser discloses a power supply on/off switching device with inrush current limiting circuitry. This device relates to regular current limiting on the AC line, and is not directed to a DC power supply.